A metal imide containing an imide anion (NH2−), for example, magnesium imide (MgNH) or the like, can be changed into magnesium hydride and a magnesium amide through a reaction with hydrogen without changing the basic structure. Since this change is reversible, metal imides have attracted attention as a precursor of a hydrogen storage material (NON-PATENT DOCUMENT 1) in recent years.
In addition, for example, it is well known that an imide compound such as EuNH accelerates catalytic reactions of olefins as a strong base, such as an isomerization reaction (NON-PATENT DOCUMENT 2). However, since a metal imide generally has extremely high reactivity, there arises a problem in that the metal imide decomposes as soon as it is left in the atmosphere.
On the other hand, among calcium aluminosilicates having CaO, Al2O3, and SiO2 as constitutional components, there is substance whose mineral name is mayenite. Compounds having the same type of crystal structure as the crystal structure of mayenite are referred to as “mayenite-type compounds”. Mayenite compounds can be generally synthesized by mixing CaCO3 and Al2O3 raw materials and then heating the mixture at a high temperature (1,300° C.).
Mayenite-type compounds have a typical composition represented by 12CaO.7Al2O3 (hereinafter, referred to as “C12A7”), and its unit cell is composed of bimolecular C12A7. That is, it can be represented by a composition formula of 2(12CaO.7Al2O3)═Ca24Al28O66, in which two oxygen ions out of 66 oxygen ions are clathrated in the form of “free oxygen” in a space of a cage formed by the crystal skeleton (NON-PATENT DOCUMENT 3). The resultant chemical formula is represented by [Ca24Al28O64]4+.2O2−.
In a mayenite-type compound, Ca constituting the above-mentioned representative composition formula may be partially or entirely substituted with at least one or more typical metal elements or transition metal elements, which are selected from the group consisting of Li, Na, K, Mg, Sr, Ba, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ir, Ru, Rh, and Pt. In addition, Al constituting the above-mentioned representative composition formula may be partially or entirely substituted with at least one or more typical metal elements or transition metal elements, which are selected from the group consisting of B, Ga, C, Si, Fe, and Ge. Further, O constituting the above-mentioned representative composition formula may be partially or entirely substituted with at least one or more typical elements or metal elements, which are selected from the group consisting of H, F, Cl, Br, and Au.
Since free oxygen ions included in mayenite are present in the cage, the oxygen ions are prevented from reacting directly with the external atmosphere. However, in the year of 2000, the present inventors found a method for generating O− and O2−, which are active oxygen species, in a cage by subjecting raw materials to a solid phase reaction under conditions of a controlled atmosphere and temperature (PATENT DOCUMENT 1).
After the year of 2003, the present inventors clarified that free oxygen ions included in mayenite can be substituted with various anions. Particularly, all of the free oxygen ions can be substituted with electrons by holding C12A7 in a strong reducing atmosphere. C12A7 in which the free oxygen ions are substituted with electrons can be represented by a chemical formula of [Ca24Al28O64]4+ (e−)4 (hereinafter referred to as “C12A7:e”). In addition, a substance containing electrons substituted for anions as described above is called an electride, and the electride features having a good electron conductivity (NON-PATENT DOCUMENTs 4 and 5).
The present inventors also found C12A7:e that is a conductive mayenite-type compound, 12SrO.7Al2O3 that is a compound of the same type as C12A7, a mixed crystal compound of C12A7 and 12SrO.7Al2O3, and a production method thereof (PATENT DOCUMENT 2).
In addition, the present inventors found that C12A7:e having conduction electrons at a concentration of 1×1019/cm3 or more and a compound of the same type as C12A7 can be obtained by (a) a method of annealing a C12A7 single crystal including oxygen ions (hereinafter referred to as “C12A7:O”) at high temperature in an alkali metal or alkaline earth metal vapor, (b) a method of ion-implanting inactive ions into a C12A7 single crystal, or (c) a method of direct solidification from a melt of a C12A7 single crystal in a reducing atmosphere (PATENT DOCUMENT 3).
Moreover, the present inventors have succeeded in obtaining C12A7:e, which exhibits metallic electrical conductivity, by annealing a C12A7:O single crystal in a titanium metal (Ti) vapor, and have filed a patent application related to an invention regarding a production method of C12A7:e and use thereof as an electron emission material (PATENT DOCUMENT 4).
Since electrons clathrated in C12A7:e are loosely bonded in a cage of a crystal skeleton of cations, these electrons can be extracted to the exterior by applying an electrical field or by employing chemical methods. On the basis of an idea that those electrons extracted to the exterior can be used in a reductive reaction, the present inventors have invented a method of producing secondary alcohol and diketone compounds by reducing ketone compounds by the electrons clathrated in C12A7:e and have filed a patent application related to the method (PATENT DOCUMENT 5).
Moreover, a patent application related to an invention regarding a mayenite-type compound in which Al is partially substituted with Ga or In has also been filed (PATENT DOCUMENT 6), and such a mayenite-type compound is suitable as an electrode material requiring high-temperature heat treatment, such as a PDP protective film material or a charge injection material in an organic EL device. A patent application related to an invention in which a C12A7 compound having a hydride concentration of 1×1018/cm3 or more can be obtained by a heat treatment under a hydrogen atmosphere has also been filed (PATENT DOCUMENT 7).
On the other hand, an attempt in which C12A7:O is subjected to a heat treatment in an ammonia gas stream to introduce a nitrogen species into the cage has been reported. For example, Boysen et al. have obtained nitrided mayenite by treating C12A7:O in an ammonia gas stream at 950° C. for 10 hours (NON-PATENT DOCUMENTs 6 to 8). The content of nitrogen in the obtained samples was within a range of 0.6 to 1.2% by weight, and it has been reported that the introduced nitrogen species was a monovalent amide anion (NH2−) from the results of neutron diffraction analysis.
Similarly, Polfus et al. have reported that C12A7:O has been subjected to a nitriding treatment at 950° C. (NON-PATENT DOCUMENT 9), and using X-ray photoelectron spectroscopy (XPS) and gas phase mass spectrometry (GP-MS), that NH2− is incorporated into the cage of C12A7. Further, Polfus et al. have mentioned that not only is NH2− incorporated into the mayenite cage but also the oxygen ions in the skeleton are partially substituted with trivalent nitrogen ions (N3−).    [PATENT DOCUMENT 1] Japanese Unexamined Patent Application, First Publication No. 2002-003218    [PATENT DOCUMENT 2] Japanese Unexamined Patent Application, First Publication No. 2005-314196    [PATENT DOCUMENT 3] Republished Japanese Translation No. WO2005/000741 of the PCT International Publication for Patent Applications    [PATENT DOCUMENT 4] Republished Japanese Translation No. WO2007/060890 of the PCT International Publication for Patent Applications    [PATENT DOCUMENT 5] Japanese Unexamined Patent Application, First Publication No. 2008-214302    [PATENT DOCUMENT 6] Japanese Unexamined Patent Application, First Publication No. 2009-203126    [PATENT DOCUMENT 7] WO2010/090266    [NON-PATENT DOCUMENT 1] U. Ash-Kurlander, G. E. Shter, S. Kababya, A. Schmidt, and G S. Grader, J. Phys. Chem. C, 117, 1237-1246, (2013)    [NON-PATENT DOCUMENT2] T. Baba, G J. Kim, Y. Ono, J. Chem. Soc., Faraday Trans., 88, 891-897, (1992)    [NON-PATENT DOCUMENT 3] H. B. Bard, T. Scheller, N. Jahrb Mineral Monatsh, 547, (1970)    [NON-PATENT DOCUMENT 4] S. Matsuishi, Y Toda, M. Miyakawa, K. Hayashi, T. Kamiya, M. Hirano, I. Tanaka and H. Hosono, Science, 301, 626-629, (2003)    [NON-PATENT DOCUMENT 5] S. Matsuishi, T. Nomura, M. Hirano, K. Kodama, S. Shamoto and H. Hosono, Chemistry of Materials, 21, 2589-2591, (2009)    [NON-PATENT DOCUMENT 6] H. Boysen, I. Kaiser-Bischoff, M. Lerch, Diffusion Fundamentals, 8, 2-1-2-8, (2008)    [NON-PATENT DOCUMENT 7] H. Boysen, I. Kaiser-Bischoff, M. Lerch, S. Berendts, A. Borger, D. M. Trots. M. Hoelzel, A. Senyshyn, ZEITSCHRIFT fur RISTALLOGRAPIIIE suppl., 30, 323-328, (2009)    [NON-PATENT DOCUMENT 8] H. Boysen, I. Kaiser-Bischoff, M. Lerch, S. Berendts, M. Hoelzel, A. Senyshyn, Acta Physica Polonica A, 117, 38-41, (2010)    [NON-PATENT DOCUMENT 9] J. M. Polfus, K. Toyoura, C. H. Hervoches, M. F. Sunding, I. Tanaka, R. Haugsrud, Journal of Material Chemistry 22, 15828-15835, (2012)